Tuneable structural and optical properties of inorganic mixed halide perovskite nanocrystals

Highly fluorescent cesium lead-based (CsPbX₃, X═Br, Cl, I) inorganic metal halide perovskites semiconductors have gained immense popularity in the last decade due to the economic and straightforward fabrication techniques involved in these materials along with their excellent electrical and optoelectronic properties. Cesium lead halide nanocrystals are well known for their fluorescence in the visible region with extremely high internal quantum efficiencies; thus making them highly suitable for the fabrication of efficient light-emitting diodes, transistors and photodetectors. Although perovskite nanocrystals (NCs) are more fluorescent compared to their bulk counterpart, there have been very few reports on the synthesis and characterization of CsPbX₃ perovskite NCs. In this work, we have synthesized and investigated the CsPbBr₃ and CsPbBr₂I NCs to understand the fundamental optoelectronic properties and structural integrity in mixed halide perovskite NCs. We have estimated ~10 nm average particle size of CsPbBr₃ nanocrystals from the high-resolution transmission electron microscopy (HRTEM) while CsPbBr₂I has ~16 nm average particle size with slightly higher polydispersity. Most interestingly, we do not observe any phase segregation of bromide and iodide ions in mixed halide perovskite quantum dots due to finite size effect. This is also confirmed by the energy dispersive X-ray spectroscopy (EDS) mapping data. However, CsPbBr₃ nanocrystals are relatively more stable than the mixed halide perovskite nanocrystals due to fewer defects. Anomalous behavior is observed in the photoluminescence intensity with the variation of precursor concentration indicating a complex nature nanoparticle synthesis.